Increasing the filler content of paper can provide the paper maker with numerous benefits, including savings in raw material costs, enhanced optical properties, and better print quality. There are, however, limits to the amount of filler that can be substituted for paper-making fiber. When the filler levels approach and exceed about 20 percent, paper can suffer losses in strength, stiffness and sizing. Unmodified fillers, such as clay, titanium dioxide and calcium carbonate, are known to have a detrimental effect on strength and sizing. Additionally, increasing the concentration of filler in the paper-making furnish results in increased size agent demand to maintain the desired hydrophobicity, water repellency, in the finished paper.
Sizing agents are generally added to cellulose fibers during the paper-making process to impart hydrophobicity to the paper. Resistance to liquid penetration is necessary to prevent the paper from breaking down when the paper is passed through a size press starch solution prior to drying. Resistance to liquid penetration is also necessary so that print quality of the paper can be maintained when printing ink is used on the surface of the paper. In particular, poor sizing efficiency is associated with the use of alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) sizing agents and calcium carbonate pigments, particularly in highly filled alkaline papers or neutral papers. It is believed that the sizing agents may be lost or rendered ineffective from the furnish due to a disproportionate fraction of a sizing agents being absorbed on the high surface area filler in the furnish. Therefore, the effectiveness of the sizing agent is reduced and the cost of the paper making process is increased due to an increase in sizing agent demand. In addition, strength properties decrease as filler levels increase, impacting negatively on the paper making operation, as well as the use of the paper as a final product. Consequently, in circumstances where increasing the filler content would be advantageous, associated sizing problems continue to occur affecting paper quality, machine performance, machine runability, and end use functionality.
The mechanism by which AKD, ASA, rosin or modified rosin agents impart hydrophobicity to cellulose fibers is somewhat controversial. However, it is generally accepted that when synthetic sizing agents, such as AKD or ASA, are used in neutral or alkaline and rosin or modified rosin agents in acidic paper-making processes, there remains a need for reducing the detrimental effects of the use of such agents on the paper-making process as well as the physical properties of the final sheet.
What is required is an inorganic base filler material that can be employed in a neutral, alkaline or acidic-making process where either an AKD or an ASA sizing agent is employed while the detrimental effects on the paper-making process and the physical properties of the final sheet are minimized.
It is therefore an object of the present invention to provide a filler that is useful in improving the sizing of neutral, alkaline and acid papers. Another object of the present invention is to provide a neutral and alkaline paper having improved sizing characteristics. Another object of the present invention is to provide a filler material that improves sizing when modified with an acrylic copolymer. A further object of the present invention is to provide an improved sizing in systems where rosin or modified rosin sizing agents are employed. Still a further object of the present invention is to provide a finished paper sheet having improved printability. These and other objects will further be disclosed and apparent in the Detailed Description of the present invention that follows.